1. Field of the Invention
The present invention relates to an off-shore seismic prospection method using one or more vibrators transmitting signals with phase variation.
2. Description of the Prior Art
Land seismic prospection methods are known comprising the transmission in the ground for several seconds of a vibratory signal whose frequency varies continuously inside a frequency band, reception by sensors of the signals reflected by underground reflectors and recording of the signals received. Because of the duration of the transmission, the signals picked up at each moment are combinations of signals reflected back by reflectors situated at very different depths. The image of the different reflectors of the sub-soil can only be found by processing the signals picked up including correlation thereof with the transmitted signals. The result of the processing is identical to that obtained by convoluting the auto-correlation function of the signal transmitted by the succession of the reflection coefficients of the different reflectors. A seismic plot is obtained which is the image of the interfaces between the different geological layers half way between the transmission and reception positions.
Such a method is described for example in the U.S. Pat. No. 2,688,124.
This method has certain drawbacks. The auto-correlation function which is obtained in this case has secondary peaks on each side of the main peak, whose amplitude is not inconsiderable. Furthermore, an interval of time at least equal to the propagation time of the transmitted waves going out to and coming back from the deepest reflector of the explored zone which will be designated by "listening interval" must be provided between two successive transmission sequences, so that the high strength signals picked up at the beginning of the corresponding recording sequence cannot mask the weakest signals coming from more distant reflectors, picked up at the end of the preceding recording sequence. The interruptions required in transmission during a relatively long listening time interval result in limiting the energy transmitted. They are particularly troublesome in offshore seismic prospection where, because of the continued advance of the ship towing the transmission-reception assembly, the difference between the successive positions where the sequences occur is considerable, which limits the spatial resolution of the recordings.
Another method is also known in which, for suppressing the listening time intervals, two signals obtained by linear scanning of the same frequency band are caused to alternate, one in the increasing direction and the other in the decreasing direction. It provides continuous transmission and recording but has the drawback of creating considerable correlation "noise". Such a method is described in the U.S. Pat. No. 3,413,596.
Other known land seismic prospection methods include the use of vibratory sources transmitting signals obtained by modulating a carrier signal by a binary signal or pseudo-random code formed of a sequence of elements able to take on two logic values 0 or l. The order of succession of these values is chosen so as to have a random character. A logic "1" leaves the sinusoidal signal unchanged. A logic "0" inverts the phase thereof.
The code is of the maximum length type, i.e. any succession of n successive bits can only be repeated identically after a sequence of (2.sup.n -1) bits. The transmitted signal is formed of a succession of identical cycles whose duration is fixed by the frequency of the carrier signal and by the number of terms of the chosen code. The duration of the signal transmitted is chosen fairly long so as to increase the transmitted energy.
Such methods are described for example in the U.S. Pat. Nos. 3,234,504 or 3,264,606.
The prior methods are adapted to land based seismic prospection where the vibratory source is moved discontinuously from one point to another of a succession of positions in a seismic profile plane to be studied.
Since the seismic source is fixed for each transmission or each transmission sequence, as well as the points of the underground reflectors reflecting the energy which is picked up by the receivers, theoretically no limit is imposed on the transmission duration.